Fine-Grained Sediment Dynamics and Morphodynamics in Tide-Dominated Channels

Abstract

Rivers supply the bulk of sediment to the world ocean and are, therefore, crucial in building the foundation of coastal ecosystems and communities. As rivers approach the ocean, sediment dispersal can become dominated by tidal processes, and much sediment is trapped within delta distributary channels. Sediment that is discharged from a river mouth is not necessarily lost to the ocean. Instead, much of this sediment migrates along the coast before depositing back onshore. While coastal environments have been intensely studied, much remains to be learned about fine-grained sediment dispersal, specifically the impact of channel-network morphology on flux pathways and morphologic development in tidal environments, which is the focus of this work. Within tide-influenced deltas, the partitioning of water and sediment flux between distributaries is often disproportionate to channel size, which influences the morphologic development of river deltas. While along-channel controls of sediment flux have been well studied in single-channel rivers and estuaries, the partitioning of flux remains incompletely understood in deltas with multiple distributaries subjected to bidirectional tidal flow. Research presented here shows that channel depth impacts the vertical distribution of sediment, and, as a result, complex bathymetry drives sediment-flux partitioning at a channel bifurcation. Water velocity and suspended-sediment properties were measured at a transect upstream of a bifurcation in the Mekong River Delta over two semidiurnal tidal cycles while the region was a seasonal tidal river. Suspended particles were predominately microflocs, and the smallest particles (<40 μm) dominated the mass settling flux, rather than the less frequent large microflocs and macroflocs. Additionally, strain rates and sediment fluxes were tidally symmetric within the deep subchannel of the bifurcating distributary but ebb dominant within the shallow subchannel. Ebb dominance promoted sediment-induced stratification, and thus sediment import upstream from the shallow distributary. Sediment-induced stratification occurred at concentrations >400 mg L-1. Comparing these results to those from previous studies at this site indicate that sediment-flux partitioning is reversed when estuarine conditions persist at the bifurcation, and that sediment delivery to downstream distributaries is reliant on both channel depth and estuarine-intrusion length. These conclusions are applicable to other tidally influenced deltas that have large supplies of fine-grained sediment, low channel sinuosity, and distributaries of similar lengths. In these deltas, the predicted global increase in sediment trapping may be disproportionately large in relatively shallow channels. Sediment that discharges from the largest tropical rivers, like the Mekong and Amazon, is commonly transported through and deposited in mangrove forests. Sedimentary research has traditionally focused on characterizing flux patterns within mangrove forests that are associated with single rivers, nearshore environments, or terminating tidal channels. However, flows within the most extensive mangrove forests tend to be driven through channels that connect to multiple estuaries or other channels, and consequently experience tides with different temporal lags. To characterize how connectivity impacts sediment transport in the world’s largest continuous mangrove stand, in-situ observations of water and sediment flux were obtained in two tidal channels near the Amazon River. The low-connectivity channel connects to one estuary whereas the high-connectivity channel connects two estuaries. Results indicate that channel connectivity controls the amount of sediment stored in channels that is available to supply mangrove forests. Although tidal phase-duration asymmetry is flood dominant in both channels, velocity asymmetry becomes ebb dominant during tidal cycles when the mangrove flats are inundated. Additionally, tidal elevation and water velocity are not in quadrature for the high-connectivity channel, as they are for the low-connectivity channel. As a result, the high-connectivity channel acts as a conduit for sediment from one estuary to another rather than simply importing and retaining sediment like the low-connectivity channel. Therefore, high-connectivity channels are expected to distribute point-source sediment along coastlines and provide sediment to estuaries and channels that may not have their own direct source. The coastline southeast of the Amazon River mouth is composed of a series of estuaries and mangrove-vegetated peninsulas, each thought to have formed by repeated sand-bar emergence and subsequent back-barrier infilling. The high-connectivity channel initially formed ca 2 ka as an open lagoon behind a sandy barrier island. Understanding how back-barrier environments infill and evolve is necessary to predict how they will respond to future changes in sea level and sediment supply. With this motivation, in-situ observations of sediment flux and sedimentary signatures from this Amazonian tidal-channel system are interpreted to create a conceptual model of morphologic evolution in a macrotidal back-barrier environment. This model applies to other environments that are similarly rich in fine-grained sediment, vegetated by mangroves, and incised by tidal channels with multiple outlets. Results indicate that within a high-connectivity back-barrier tidal channel, tidal processes dominate sedimentation and morphological development. Shallow cores (<60 cm) exhibited millimeter-scale tidalites composed of sand and mud. High-connectivity channels allow tidal propagation from multiple inlets, and in this case, the converging flood waves promote delivery of sediment fluxing through the system to the mangrove flats in the convergence zone. Sediment preferentially deposits in regions with adequate accommodation space and dense vegetation, and in these zones, bed sediment grain size is slightly finer than that transiting through the system. The greatest sediment-accumulation rates (3 to 4 cm yr-1), calculated from steady-state 210Pb profiles, were found in the convergence zone near the mangrove-channel edge. As tidal flats aggrade vertically and prograde into the channels, accommodation space diminishes. In effect, the channel’s narrowest stretch is expected to migrate along the path of net-sediment flux toward regions with more accommodation space until it reaches the tidal-convergence zone. The location of recent preferential infilling is evidenced by relatively rapid sediment-accumulation rates, finer sediment, and significant clustering of small secondary tidal channels. These findings shed light on how sediment transported through vegetated back-barrier environments is ultimately preserved and how evidence preserved in surface morphology and the geological record can be interpreted.